If using the red and white gas carts, make sure you are using compressed air and NOT METHANE gas which is highly flammable.

Discussion:

There are at least 3 variations of this demonstration. The below photos show the various appropriates.

The First organ pipe has a slide-able card, located at the mid point of the air column, with a hole only on one end. With the card pushed in, thus the hole is within the organ pipe, and with low to moderate air flow. The organ pipe acts like an open-open tube resonator of length L. When the card is pulled out, there by closing the organ pipe at the mind point. The pitch of the sound then drops by a factor of 4 and now acts like a open-closed tube resonator to L/2.